The present invention relates generally to optical lithography, and particularly to optical microlithography crystals for use in optical photolithography systems utilizing vacuum ultraviolet light (VUV) wavelengths below 200 nm, preferably below 193 nm, preferably below 175 nm, more preferably below 164 nm, such as VUV projection lithography systems utilizing wavelengths in the 157 nm region.
Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light below 200 nm provide benefits in terms of achieving smaller feature dimensions. Such systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits with smaller feature sizes. Current optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm wavelengths, but the commercial use and adoption of vacuum ultraviolet wavelengths below 200 nm, such 193 and 157 nm has been hindered by the transmission nature of such vacuum ultraviolet wavelengths in these VUV excimer laser regions through optical materials and the surfaces of the optical materials. For the benefit of vacuum ultraviolet photolithography in the 157 nm region such as the emission spectrum VUV window of a F2 excimer laser to be utilized by the semiconductor industry in the manufacturing of integrated circuits there is a need for optical lithography crystal surfaces with beneficial angstrom level roughness that can be economically and efficiently manufactured.
There is a need for an economical and efficient means for forming angstrom level roughness high quality surfaces in optical lithography fluoride crystal surfaces, such as the surface of calcium fluoride crystals that transmit 157 nm light. The polishing of calcium fluoride lithography crystals is complicated by calcium fluoride""s high thermal expansion and low hardness. Prior art methods of polishing calcium fluoride VUV optical lithography surfaces are time consuming and inefficient to produce from an industrial manufacturing respect with polishing process times ranging from days to weeks. Japanese Patent Application 11 [1999]-87808 (Mar. 30, 1999) of Nikon Corp. describes a Method for Manufacturing Optical Elements for ArF Excimer Laser uses. As noted in this Nikon Corp. Japanese Patent 11-87808, optical elements for optical systems using a high-pulse ArF 193 nm excimer laser are manufactured by polishing fused silica glass with CeO2 polishing agents and by polishing fluorite with diamond powder polishing agents. Cerium polishing of fluorite optical lithogaphy fluoride crystal surfaces has been avoided by the prior art due to cerium oxide contamination concerns that any cerium molecules/atoms/ions incorporated into the optical lithography fluoride crystal surface by polishing and then exposed to the highly energetic high fluence levels produced by 157 nm and 193 nm excimer lasers and used in optical lithography systems will strongly absorb the VUV light with the VUV absorbing cerium then damaging and corrupting the fluoride crystal structure and producing further detrimental VUV absorptions.
The present invention overcomes problems in the prior art and provides a polished finished optical lithography fluoride crystal surface that can be used to improve the lithographic manufacturing of integrated circuits with VUV wavelengths.
The invention includes a method of making a below 200 nm vacuum ultraviolet optical microlithography lens element. The method includes providing a fluoride crystal, providing a cerium oxide polish, and polishing the fluoride crystal with said cerium oxide polish to provide an optical microlithography element.
In another aspect, the invention includes a method of making a below 200 nm optical microlithography element preform. The method of making a below 200 nm optical microlithography element preform includes providing a fluoride crystal, providing a cerium polish, and polishing the fluoride crystal with the cerium polish to provide a microlithography element polished preform.
In a further embodiment the invention includes a method of making a below 170 nm optical microlithography preform. The method of making the below 170 nm optical microlithography preform includes providing a calcium fluoride crystal having a 157 nm internal transmission greater than 95%/cm and providing a cerium polish. Preferably the cerium polish includes cerium oxide particles in an acidic polishing environment. The method includes polishing the calcium fluoride crystal with the cerium polish to provide a VUV microlithography polished preform surface.
In a further aspect the invention includes a method of making a 157 VUV optical element preform. The method includes providing a calcium fluoride crystal with a 157 nm internal transmission greater than 95%/cm. The method includes providing a cerium means for polishing the crystal into a 157 nm VUV optical element preform with a polished preform surface roughness less than 5 angstroms. The method includes polishing the calcium fluoride crystal with the cerium polishing means to a surface roughness less than five angstroms.
In another aspect, the invention includes a method of making a below 200 nm optical microlithography element preform. The method of making a below 200 nm optical microlithography element preform includes providing a fluoride crystal, providing an aqueous polish, and polishing the fluoride crystal with the aqueous polish to provide a microlithography element polished preform.